1. Field of the Invention
The present invention relates to digital data communication networks and, more specifically, to an apparatus and method for testing the ability of such a network to appropriately enter and resolve contests among elements on the network for control of the data bus.
2. Description of the Related Art
A digital data communication network such as an Integrated Service Digital Network ("ISDN") is shown in FIG. 2. The ISDN includes a plurality of terminal equipment units ("TEs") 1 and a Digital Service Unit ("DSU") 2. According to the International Telegraph and Telephone Consultative Committee standard I.430 ("CCITT-I.430"), an ISDN can accomodate a maximum of 8 TEs. Each of the TEs 1 is coupled to a DSU 2 by a data bus comprising a transmit line ("T line") and a return line ("R line") by which TEs 1 communicate with DSU 2. Each TE selectively transmits digital data to DSU 2 through the T line, and receives digital data from DSU 2 through the R line.
Digital data is communicated by each of the TEs 1 in the form of frames. Frame structures according to CCITT-I.430 are illustrated diagrammatically in FIG. 3/I.430 of the CCITT standard. An example of such frame structures is shown in FIG. 3, which illustrates a frame A and a frame B. Frame A communicates data from a given TE 1 to DSU 2, and includes a plurality of D-channels. Each D-channel comprises control data or a control bit, for example, the data "0" or "1", to indicate that the given TE desires or does not desire control of the data bus. The control data "0" generally indicates that control is desired whereas the control data "1" indicates control is not desired. Frame B communicates data from DSU 2 to TEs 1, and includes a plurality of E-channels corresponding to the D-channels. When a TE 1 transmits data in the D-channels of frame A to DSU 2 through the T line, DSU 2 returns this same data to corresponding ones of the E-channels of frame B and communicates the E-channels through the R line. Each TE continuously monitors the data in the E-channels of frame B.
When a TE does not require control of the data bus, it transmits the data "1" as the control data in the D-channels of frame A through the T line. DSU 2 returns the data "1" from the D-channels along the corresponding E-channels and transmits the E-channels in frame B through the R line.
When a TE requires control of the data bus, it transmits the data "038 as the control data in the D-channels of frame A. DSU 2 returns the data "0" from the D-channels through the corresponding E-channels in frame B through the R line. The TEs receive this E-channel data and compare it with the D-channel data transmitted. Upon determining that the control data of the D-channel and E-channel match, and upon determining from frame B that only that TE 1 transmitted a control data "0," that TE obtains control of the data bus and can begin or continue transmitting data on the data bus.
If more than one TE 1 transmits a control data "0" indicating an intention to begin transmitting simultaneously, there is contention for control of the data bus. Each of the TEs contending for control then operates in accordance with a set of priority rules such as those set forth in CCITT-I.430 to avoid and resolve this contention. Resolution is obtained by selecting one of the TEs to which control of the bus is given while the other TEs contending for control of the bus are put in a wait condition or state. While in the wait state, the other TEs 1 can not use the D-channels of frame A. Application of the priority rules also prevents malfunction of the contention and resolution processes.
When testing the ability of the TEs coupled to DSU 2 to identify a bus control contest and to resolve the contest in accordance with priority rules such as CCITT-I.430, it is generally necessary for each of TEs 1 to transmit the control data, e.g., the data "0", simultaneously. There has been a need for apparatus and methods to cause this simultaneous transmission of data from the TEs.